– Remove fish skin with a boning or paring knife. To do this, put your fillet on the cutting board with the skin side down, and make a cut to separate a small flap of the skin in one of the corners closest to you. Grab the flap, and slide the knife between the skin and the fillet. Make sure to slide the knife all the way under the fillet, trying not to leave any of the fish meat on the skin.
– Cut the fish in small ⅓-in. cubes and place in a glass bowl. Season with salt and add the lime juice. Cover the bowl with plastic wrap and place in the fridge, and marinate for at least 30 minutes. Meanwhile, prepare the rest of the ingredients.
– Finely chop the tomatillos, onion, peppers, and olives. Mix in a large bowl with the cilantro, Mexican oregano, and ground black pepper.
– After 30 minutes, remove fish from the fridge and gently toss with the ingredients into the large bowl. Just before serving, dice the avocado and toss into the ceviche, drizzle the olive oil, and taste to add more salt if needed.